Sequel as applied to a work of literature is a modern usage, though. Yes it means "to follow" but it has been meant to mean a story that follows and continues another since the early 1500s. And for us here in this thread we are using the sense of "continue" to mean being set chronologically after another. The portmanteau "prequel" exists for a reason--to delineate stories created chronologically after another but set before them from stories created chronologically after another but set after them. If people didn't want to delineate between prequels and sequels based on their chronological order in a story, they wouldn't be using the word prequel! It exists for a reason. In fact, yet another term, interquel, has been coined for stories created chronologically after both sequels and prequels but set to take place in between them (The Clone Wars CGI cartoon, for instance, to use a Star Wars reference).

So, let's set semantics aside and go with what the topic creator wants to discuss, as set forth in the first post of the topic, which is a sequel set after three other movies in a storyline. We can call it that from now on--we'll use the acronym SSATOMIAS.

Anyway, I read yesterday that apparently The Wolverine is being set after X3, so it is presumably a SSATOMIAS.

EDIT: Let me put this another way--I could argue etymologically that the term "subway" is so vague it could be applied to numerous things. An underpass is a "subway". A tunnel through a mountain that has a road on top of it is a "subway". However, if someone had quite clearly laid out in context that they were talking about a series of tunnels under a city through which trains travel, it would seem quite pedantic for me to argue that a subway is any path that goes under another path.

I'll leave it to everyone to imagine what it would be like if, when people said "submarine", it conjured up every image from a fish to a sea sponge, or every man-made construct from a bathysphere to a diving suit. They're all "under the water", after all.